This invention relates generally to methods and apparatus for computed tomographic (CT) imaging and other scanning imaging systems, and more particularly, to methods and apparatus for decreasing image artifacts and for retaining spatial resolution in systems providing increased volume coverage with limited data acquisition hardware.
To reduce the total scan time required for multiple slices, a xe2x80x9chelicalxe2x80x9d scan may be performed. To perform a xe2x80x9chelicalxe2x80x9d scan, the patient is moved in a z-axis direction synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan.
In at least one known CT imaging system, the detector array is segmented into a plurality of rows of detector elements, each row extending in an x-direction and defining an x-y xe2x80x9cslicexe2x80x9d out of a volume. Such imaging systems can acquire and process a plurality of slices of projection data to construct a plurality of images. Each of the reconstructed images corresponds to a different slice though the volume. Such CT imaging systems are referred to as xe2x80x9cmultislicexe2x80x9d systems. Multislice systems provide more data available for reconstructing an image. In one exemplary embodiment of a multislice imaging system, N rows, e.g., N slices, of projection data are acquired for each view angle.
At least one known CT imaging system utilizes a detector array and a xe2x80x9cdata acquisition systemxe2x80x9d (DAS) for collecting image data. The detector array includes detector elements that individually produce an analog intensity signal representing impinging x-ray energy. The analog signals are then converted by the DAS to digital signals, which are used to produce image data.
In many applications of CT imaging systems such as cardiac imaging, it is desirable to increase the volume of an object imaged during a gantry rotation. One method for increasing imaging volume is to increase z-axis detector coverage, for example, by increasing the number of detector rows. To accommodate a larger number of detector rows without substantially increasing the size and complexity of the DAS, adjacent groups of detector elements in a detector row can be ganged. xe2x80x9cGanging groups of detector elementsxe2x80x9d (also called xe2x80x9cganging detector channelsxe2x80x9d) refers to combining outputs of more than one detector cell adjacent one another in the x-direction. A detector having ganged detector elements requires less bandwidth than the same size detector array having separate detector outputs. Each ganged group of detector elements produces an output that requires only a communication path or channel through the DAS equivalent to that required by a single, unganged detector output. For example, a ganged group of detector elements produces data that can be sent via one multiplexed data slot position. Spare detector channels of the DAS can then be used to increase the number of detector rows from which data is acquired. When increased volume coverage is not needed, the channels and detector elements can be xe2x80x9cungangedxe2x80x9d or used individually. In this mode, the DAS accommodates the same amount of data, but processes fewer detector rows, so that less z-axis coverage is provided.
Ganging of adjacent detector elements in a detector row allows a hardware-limited imaging system to provide increased volume coverage. However, computer simulations of phantom scans has shown that ganging of detector elements in this manner results in a significant loss of spatial resolution and in increased aliasing artifact levels. Attempts to improve spatial resolution by boosting the image reconstruction kernel have resulted in worsened aliasing.
There is therefore provided, in one embodiment of the present invention, a method for imaging an object with a scanning imaging system having a rotating gantry, a multislice detector array on the rotating gantry having parallel rows of detector elements, the parallel rows defining an x-direction, and an x-ray source on the rotating gantry configured to emit a fan-shaped x-ray beam in an x-y plane through an object to be imaged and towards the multislice detector array, the multislice detector array and the fan-shaped beam together defining an first iso-channel (or xe2x80x9cunganged iso-channelxe2x80x9d) of the multislice detector array when the scanning imaging system is utilized for scanning without ganging groups of detector elements. The method includes steps of ganging groups of adjacent detector elements in the x-direction; scanning an object with the scanning imaging system using the ganged groups of adjacent detector elements to acquire projection data; and reconstructing an image of the object utilizing the acquired projection data and an adjusted iso-channel of the multislice detector array different from the first iso-channel.
Another embodiment of the present invention provides a method for imaging an object with a computed tomography (CT) imaging system having a rotating gantry, a multislice detector array on the rotating gantry having parallel rows of detector elements, the parallel rows defining an x-direction, and an x-ray source on the rotating gantry configured to emit a fan-shaped x-ray beam in an x-y plane through an object to be imaged and towards the multislice detector array, the multislice detector array and the fan-shaped beam together defining an first iso-channel of the multislice detector array when the CT imaging system is utilized for scanning without ganging groups of detector elements. The method includes: ganging groups of adjacent detector elements in the x-direction; scanning an object with the CT imaging system using the ganged groups of adjacent detector elements to acquire projection data; and reconstructing an image of the object utilizing the acquired projection data and an adjusted iso-channel of the multislice detector array different from the first iso-channel; wherein reconstructing an image of the object includes filtering and backprojecting the acquired projection data without interpolation.
Yet another embodiment of the present invention provides a method for imaging an object with a computed tomography (CT) imaging system having a rotating gantry, a multislice detector array on the rotating gantry having parallel rows of detector elements, the parallel rows defining an x-direction, and an x-ray source on the rotating gantry configured to emit a fan-shaped x-ray beam in an x-y plane through an object to be imaged and towards the multislice detector array, the multislice detector array and the fan-shaped beam together defining an first iso-channel of the multislice detector array when the CT imaging system is utilized for scanning without ganging groups of detector elements. The method includes: ganging groups of adjacent detector elements in the x-direction; scanning an object with the CT imaging system using the ganged groups of adjacent detector elements to acquire projection data; and reconstructing an image of the object utilizing the acquired projection data and an adjusted iso-channel of the multislice detector array different from the first iso-channel, wherein the reconstructing includes performing a backprojection on the acquired projection data, and selecting, as the adjusted iso-channel, an iso-channel in the backprojection offset from the first iso-channel by an amount effective to reduce artifacts in the reconstructed image.
Still another embodiment of the present invention provides a method for imaging an object with a computed tomography (CT) imaging system having a rotating gantry, a multislice detector array on the rotating gantry having parallel rows of detector elements, the parallel rows defining an x-direction, and an x-ray source on the rotating gantry configured to emit a fan-shaped x-ray beam in an x-y plane through an object to be imaged and towards the multislice detector array, the multislice detector array and the fan-shaped beam together defining an first iso-channel of the multislice detector array when the CT imaging system is utilized for scanning without ganging groups of detector elements. The method includes ganging groups of adjacent detector elements in the x-direction; scanning an object with the CT imaging system using the ganged groups of adjacent detector elements to acquire projection data; and reconstructing an image of the object utilizing the acquired projection data and an adjusted iso-channel of the multislice detector array different from the first iso-channel; wherein the reconstruction includes filtering the projection data utilizing a kernel having a cutoff frequency nearly twice a Nyquist spatial sampling frequency of said multislice detector array when said groups of detector elements are ganged.
In another aspect, the invention provides a scanning imaging system for imaging an object, the scanning imaging system having a rotating gantry, a multislice detector array on the rotating gantry having parallel rows of detector elements, the parallel rows defining an x-direction, and an x-ray source on the rotating gantry configured to emit a fan-shaped x-ray beam in an x-y plane through an object to be imaged and towards the multislice detector array, the multislice detector array and the fan-shaped beam together defining an first iso-channel of the multislice detector array when the scanning imaging system is utilized for scanning without ganging groups of the detector elements, the scanning imaging system configured to:gang groups of adjacent the detector elements in the x-direction; scan the object using the ganged groups of the adjacent detector elements to acquire projection data; and reconstruct an image of the object utilizing the acquired projection data and an adjusted iso-channel of the multislice detector array different from the first iso-channel.
In still another aspect, the invention provides a computed tomographic (CT) imaging system for imaging an object, the CT imaging system having a rotating gantry, a multislice detector array on the rotating gantry having parallel rows of detector elements, the parallel rows defining an x-direction, and an x-ray source on the rotating gantry configured to emit a fan-shaped x-ray beam in an x-y plane through an object to be imaged and towards the multislice detector array, the multislice detector array and the fan-shaped beam together defining an first iso-channel of the multislice detector array when the CT imaging system is utilized for scanning without ganging groups of the detector elements, the CT imaging system configured to: gang groups of adjacent the detector elements in the x-direction; scan the object using the ganged groups of adjacent detector elements to acquire projection data; and reconstruct an image of the object utilizing the acquired projection data and an adjusted iso-channel of the multislice detector array different from the first iso-channel; wherein to reconstruct the image of the object, the CT imaging system is configured to filter and backproject the acquired projection data without interpolation.
In yet another aspect, the present invention provides a CT imaging system for imaging an object, the CT imaging system having a rotating gantry, a multislice detector array on the rotating gantry having parallel rows of detector elements, the parallel rows defining an x-direction, and an x-ray source on the rotating gantry configured to emit a fan-shaped x-ray beam in an x-y plane through an object to be imaged and towards the multislice detector array, the multislice detector array and the fan-shaped beam together defining an first iso-channel of the multislice detector array when the CT imaging system is utilized for scanning without ganging groups of the detector elements, the CT imaging system configured to: gang groups of adjacent the detector elements in the x-direction; scan the object using the ganged groups of adjacent detector elements to acquire projection data; and reconstruct an image of the object utilizing the acquired projection data and an adjusted iso-channel of the multislice detector array different from the first iso-channel; wherein to reconstruct the image of the object, the CT imaging system is configured to perform a backprojection on the acquired projection data, and to select, as the adjusted iso-channel, an iso-channel in the backprojection that is offset from the first iso-channel by an amount effective to reduce artifacts in the reconstructed image.
In yet another aspect, the present invention provides a CT imaging system for imaging an object, the CT imaging system having a rotating gantry, a multislice detector array on the rotating gantry having parallel rows of detector elements, the parallel rows defining an x-direction, and an x-ray source on the rotating gantry configured to emit a fan-shaped x-ray beam in an x-y plane through an object to be imaged and towards the multislice detector array, the multislice detector array and the fan-shaped beam together defining an first iso-channel of the multislice detector array when the CT imaging system is utilized for scanning without ganging groups of the detector elements, the CT imaging system configured to: gang groups of adjacent the detector elements in the x-direction; scan the object using the ganged groups of adjacent detector elements to acquire projection data; and reconstruct an image of the object utilizing the acquired projection data and an adjusted iso-channel of the multislice detector array different from the first iso-channel; wherein to reconstruct the image, the CT imaging system is further configured to filter the acquired projection data utilizing a kernel having a cutoff frequency nearly twice a Nyquist spatial sampling frequency of the multislice detector array when the groups of detector elements are ganged.
Still another aspect of the present invention provides a processor configured to: input projection data representative of an object scanned with a scanning imaging system having ganged groups of adjacent detector elements; and reconstruct an image of the object utilizing the input projection data, wherein the reconstruction includes performing a backprojection on the acquired projection data, and selecting, as the adjusted iso-channel, an iso-channel in the backprojection offset from the first iso-channel by an amount effective to reduce artifacts in the reconstructed image.
Yet another aspect of the present invention provides a computer-readable medium having encoded thereon instructions configured to instruct a computer to: input projection data representative of an object scanned with a scanning imaging system having ganged groups of adjacent detector elements; and reconstruct an image of the object utilizing the input projection data, wherein the reconstruction includes performing a backprojection on the acquired projection data, and selecting, as the adjusted iso-channel, an iso-channel in the backprojection offset from the first iso-channel by an amount effective to reduce artifacts in the reconstructed image.
These embodiments of the present invention allow detector elements to be ganged to provide increased scan volumes, while reducing the degradation of images due to aliasing artifacts and reduced spatial resolution that would otherwise result from the ganging of detector elements.